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Transmit Problem with MCS2000 and Antenna

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SOUTH300

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I just recently re-installed a MCS2000 Model II VHF from one vehicle to another. 2008 F-150 to a 2009 F-150. I am currently using a Diamond K-400 Trunk Lip antenna mount and secured it to the bed rail. In the old truck, it worked like a dream. In the new truck, this is what happens:

On certain channels, not all, the radio will attempt to key but then stop after about one second. If you hold the PTT, it will continuously try to key. It appears that it does x-mit, because it will hit the repeater for a second. The tail sounds clear. The Transmit frequencies are in the area of 154-158 mhz. However on some channels in this range, the radio appears to transmit fine. In fact, I can hit repeaters that are 30-35 miles away, and sound clear.

Here is what I have checked:
Power and ground to the Radio are good.
The K-400 appears to be grounded good to the bed.
I ran a separate ground from the antenna to the frame, and problem still exists.
When I unscrew the antenna from the K-400 mount, the radio DOES key. (I know you are not supposed to do this)
The problem returns when the antenna is screwed back on.
I have tried two separate antennas on the K-400, one adjustable, and one non.
Sometimes when the radio is on low power, the radio will key, but will not key when it is on high power.
This leads me to believe it is an antenna issue, but I cannot figure it out.

Open for any suggestions. I do have the CPS/Tuner but no test equipment.

Thanks
south300
 

kayn1n32008

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Ditch the trunk lip mount. While it may appear to be grounded, it is a crappy mount(ALL trunk lip mounts are) use a proper L bracket, or better yet drill a hole in the roof... You will like the results with a roof mounted antenna.


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mmckenna

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Right, K400 is the antenna mount, not the antenna. Some insight into the antenna being used might help.

Mounting antennas on the truck bed rail is less than idea. It might be close enough to the cab to be reflecting power. Bad ground might be an issue. DC grounds are not the same as an RF ground. Running a ground wire to the "frame" isn't the same as providing a proper ground plane for the antenna (if it requires one).

Try moving the antenna away from the cab as far as you can. Better yet, as stated, get a permanent mount NMO up on the center of the cab roof, it'll work much better.

The fact that it keys up and stops suggests high SWR.
The fact that it works on low power reinforces this.

Also, any chance you damaged the coaxial cable while installing it? Anywhere the coax could be pinched/crimped/damaged?
 

KG4INW

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Do you have access to a wattmeter? Check to make sure you don't have high reflected power (which it sounds like).
 

SOUTH300

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The first antenna, which worked properly with this setup on my old truck is the Diamond M285NMO. The picture shows what I am currently trying...it is 18 inches long. This is the one I want to use.

The coax is practically brand new, and I inspected it to be sure. I have it as far away from the cab as I can with the amount of coax. I know the trunk lip mount is not ideal, but my truck was a company truck and I couldn't drill. With the M285NMO I needed the fold over capability for my garage also.

I can't seem to find anything I did different from the old truck to the new truck, and since they are both Fords and only a year apart, I wouldn't think anything major changed to cause this.

I have a voltmeter but not a watt meter handy. I have a friend with an SWR meter, but how would I adjust the SWR with this antenna? (Not the M285, it is adjustable)

Thanks for the help guys

South300
 

mmckenna

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Try this:

1. Remove the antenna from the mount.
2. Remove the connector from the back of the radio.
3. Use your multimeter to check for continuity between the center conductor pin of the coax connector and the outer conductor.
-there shouldn't be a connection between those two. If the meter shows one, you have a fault in the coax, connector or antenna mount. You'll have to disassemble them to find it.

Next:
With all the above still disconnected, check for continuity between the center conductor and a known good vehicle ground. There shouldn't be. If there is, disassemble until you find it.

Next:
With all the above still disconnected, check for continuity between the coax connector outer shield and the known good vehicle ground. There should be continuity.

If all that checks out, you need to try an SWR meter. You need to specifically be looking for abnormally high reflected power (high SWR).
As for how to adjust the antenna, I can't tell you since the photo you attached isn't showing up. If you can provide a link to the model on the internet, that would help.
Sounds a lot like a quarter wave VHF if it's 18 inches. Those are quite broadband and should be showing acceptable SWR on your 154-158 frequencies. It might need a bit of tuning, but it shouldn't be so out of whack to be causing the issues you are seeing.

Oh, one more thing, make absolutely sure the antenna whip center contact on the NMO antenna base is positively and without a doubt actually contacting the center point on the NMO base. I've seen some NMO antennas that didn't make good contact, and this would cause a high SWR.

Photos are worth a thousand words, so if you can try attaching a new photo, that would probably help.
 

SOUTH300

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mmckenna,

Here is the picture of the antenna. I checked the continuity between all points just as you described and all appears to be good. I also checked the NMO base and it appears to be contacting the center pin of the antenna just fine. Also, as you can see in the picture, I have the K400 mounted against the plastic bed liner. The K400 has rubber on that side of the mount, so I did not think it would be a problem. Just to be sure, I pulled back the plastic and mounted it straight to the bed, and the problem is still there.

Thanks again for your help.
south300
 

WA0CBW

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The stinger antenna and the NMO mount are pretty straight forward. As "mmckenna" said check the continuity and shorts in the coax and connectors. There isn't much else that can go wrong.
BB
 

SOUTH300

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Well, I tried something tonight that is not antenna related at all. I am using MDC1200 on a few of the VHF channels, but not all, that were giving me problems. I turned off the PTT Sidetone first to see what would happen. I did not turn off the actual MDC transmission, just the sidetone. I am transmitting the MDC at the beginning of the transmission. The problem stopped. It appears that it is transmitting fine now. I wonder if there is something wrong with the MDC board? Still stumps me though, because the problem was appearing on channels that I did not have MDC programmed on.

South300
 

mmckenna

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Interesting. Not sure what/why the MDC sidetone seems to be affecting that.

I'd still recommend testing with an SWR meter just to be sure.

I've had some of those NMO antennas with the taller "chrome nut" part not always make good contact with the center of the NMO mount. You can usually tell by holding the whip and moving it up and down. Sometimes you'll feel the whip making contact with the center pin.
 

cmdrwill

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That antenna mount is known to not work with low SWR in some applications. And that mounting is very questionable. Something most miss is the OTHER half of the antenna, the counterpoise. The counterpoise or groundplane needs to be 1/4 wave in all directions, and you do not have that.

mmckenna,

Here is the picture of the antenna. I checked the continuity between all points just as you described and all appears to be good. I also checked the NMO base and it appears to be contacting the center pin of the antenna just fine. Also, as you can see in the picture, I have the K400 mounted against the plastic bed liner. The K400 has rubber on that side of the mount, so I did not think it would be a problem. Just to be sure, I pulled back the plastic and mounted it straight to the bed, and the problem is still there.

Thanks again for your help.
south300
 

SOUTH300

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Interesting. Not sure what/why the MDC sidetone seems to be affecting that.

I'd still recommend testing with an SWR meter just to be sure.

I've had some of those NMO antennas with the taller "chrome nut" part not always make good contact with the center of the NMO mount. You can usually tell by holding the whip and moving it up and down. Sometimes you'll feel the whip making contact with the center pin.
I'm going to check the SWR. It started again. Works on low power but not in high power.
 

mmckenna

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It really sounds like SWR is too high. I agree with what cmdrwill says, that mount -might- provide a DC ground, but no way it's providing a proper RF ground.

If you really must have the antenna mounted right there and it cannot possibly go anywhere else on the truck, get a half wave VHF antenna. They will work without a ground plane under the antenna (but work better with one). I've got a VHF half wave mounted off on one corner of a UTV roll cage. Improper ground plane for a 1/4 or 5/8ths wave antenna, but tunes up nicely with the half wave.

Either way, you need to check the SWR.

Not sure if this was mentioned anywhere above, but you should also check all your DC power connections. Any connections that are damaged, corroded or not properly crimped can cause the voltage to drop when the radio transmits. Often this will present as the radio shutting off or restarting, but they can also "bonk" and stop transmitting.
 

hitechRadio

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I believe you problem would be fixed if mounted on the roof. But since it is not your truck, the correct way is not possible. Lets assume your SWR is ok. Then it sounds like RF is getting in to the control head, or some part of the radio. Try moving your control head in different locations, and see if that is the problem. If it is RF getting into radio, you may try a different location on you truck bed. Or as others have mentioned an NMO angle mount buy the hood on drivers side.

I have never seen a Motorola reset if antenna was on roof and that had high SWR.
I have however seen a radio reset with good SWR but not mounted thru roof.
I have only seen this problem with 100watt High Power Mobiles.

I assume it is a 100watt radio correct, since I believe you mentioned High Power? If so It is highly recommended you mount that antenna on the roof! Not only is this poor practice, but it can cause major antenna pattern problems. That being said, I understand you kind of have a dilemma being its you work truck, and you limited to mounting locations...good luck my friend.

FYI:

This may not have anything to do with your issue, and you may have already wired it this way. But.........
You should run the positive side of power cable direct to battery, or as close to battery as possible (some vehicles have those side posts). The negative should be grounded to chassis ground NOT the battery, I usually but not always (depends on install) keep the negative lead short and close to the radio deck. Sometimes you get lucky and find a factory ground lug nearby. If not, grind any paint off and attach negative were it cant be seen easily ex: under carpet out of foot traffic, or remove the plastic foot step and ground under there. Check and make sure you wont be drilling into anything important, wires, hoses ext.
 
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